Club drugs are a broad range of substances used in dance clubs, parties, raves, bars, and other venues. Drug formulations often contain pharmacologically active agents known as "adulterants". These substances, whose presence is unknown to users, produce undesirable effects for which MSM often take "coingestants". The use of multiple substances by MSM in high risk environments is common; even individuals who intend to use a single drug such as "ecstasy" may often receive greater than one substance because of adulterants. Adulterants, drugs, and coingestants affect human physiology and, consequently, sexual behavior and HIV risk. "Ecstasy" tablets- especially those adulterated with methamphetamine- produce impotence and prevent men from performing insertive anal intercourse. In highly sexualized environments, these MSM may engage in receptive anal sex, a behavior that increases HIV transmission risk. Viagra is paradoxically used by some MSM as a prophylaxis against HIV transmission because it promotes erection, facilitates use of a condom, and allows penetrative anal sex. Viagra also facilitates increased numbers of sexual contacts, longer periods of sexual activity, and intergenerational sex, features that may increase the risk of HIV transmission. Little is known about the phenomenon of drug, adulterant, and coingestant use. Surveys cannot examine the self-report of adulterant use, and the limited testing for adulterants in pill contents has produced suspect results. Similarly, the range of coingestants used by MSM or reasons for their use is poorly understood. Viagra may exert a bidirectional risk on HIV transmission, but no comprehensive studies have identified the outcomes of coingestant use. The specific aims are: 1) To determine the prevalence of drug, adulterant, and coingestant use based on the detection by HPLC of these substances in the urine among MSM attending venues in Provincetown, MA during summer months over a two year period; 2) To compare self-reported drug and coingestant use with the gold standard of HPLC analysis of urine for drugs, adulterants, and coingestants; and 3)To explore the association of HIV risk behaviors with drug, adulterant, and coingestant use. Before interventions can be applied to the problem of substance abuse among MSM, a rigorous definition of what comprises "drugs" used in venues of MSM social culture must be developed, as well as contexts of these substances' use. This study is important because it will develop a foundation for understanding the nature of drug, adulterant and coingestant use, as well as their impact on social behavior in a core risk group for HIV transmission. Moreover, it will compare HIV risk behaviors with biological standards of drug use. Integrating analytical laboratory science into social science methods represents an innovative interdisciplinary approach to HIV research that will provide a basis for future research on these substances.